It wasn't me, but I too follow UA's boarding policy to the letter, which does not mention pregnancy as a preboard condition.

My take on the matter...

1) UA ATTEMPTS to do 2 things with boarding...a) be efficient, and b) reward UA loyalty.

2) It is at the discretion of a GA to make an exception for "customers requiring more time" to pre-board.

3) IMHO, ths is less efficient than boarding with groups, as preboarding takes away time while waiting for you to get down and settled prior to sending others down, while if you board with your group, there is already a line, and it is moving slowly, most likely as slow as the pace for which you will need.

4) Often the people who preboard are doing it so their family (or self) can stow their luggage. UA offres seating area one boarding (which can not be bought at most mainline stations) to those who are not of status and wish to use this option.

5) Many aircraft are on "minimum turns" (30 min from block to block depending on aircraft type) and the boardtime time is less than 1/2 that. To take time out to convienence 1 family in these flights usually ends up delaying the flight a few extra min.

6) People in premium cabins and people of status get to preboard. This is a benefit that is given to them becuase of frequency (or quantity or cost) of travel. Any familly can get this status and preboard pregnant or not. While it may be pleasant for you to be given this privlidge, it is just 1 more exception to those that have earned this right.

There are clearly arguements on the other side, but my arguements is that it is actually less efficient when there are time constraints, it undermines the boarding of status passengers, it is not a disability, rather in most cases, a self imposed blessing, and that UA's SOP does not include it, rather they leave it as an exception up to the discression of the GA.

You aren't wrong to ask, but as many said, they wern't wrong to deny.